This invention relates to improvements in motor current measurement circuitry, in particular but not exclusively for use as part of a motor control circuit suitable for the control of a motor that forms part of an electric power assisted steering (EPAS) system.
It is known to provide a current measurement circuit for measuring the current flowing into and out of the ground return path, or DC link, of a multi-phase motor of an EPAS system. This can be used as part of the control circuit for a low speed sensorless motor, by which we mean a motor in which no dedicated position sensor is provided and instead a measure of current is used to estimate the motor position. By knowing the impedance of the motor when the rotor is at different positions it is possible to determine its position from the current measurement and how the current measurement changes over time. But this requires a highly accurate measurement if precise control is needed, which has proven especially problematic in the demanding environment in which EPAS systems are located, subject to high levels of EM noise in an often limited space.
Of course, current sensing circuits find application in a wide range of other circuits, and the present invention has application in other circuits where a high precision current measurement is required.
Typically, in prior art current measurement circuits for use in automotive applications the current measurement signal is derived from amplifying the potential difference, or voltage drop, across a resistor placed in series in the current path. With a motor, this typically requires the resistor to be placed in series with two phases of the motor or the DC link. This signal is amplified as required to a level suitable for direct input to an analogue to digital converter (ADC) where it is converted into a digital representation using an analogue to digital converter. The current measurement signal in the prior art is obtained using a differential amplifier whose output comprises a voltage proportional to the voltage dropped across the resistor. An example of such a circuit is known from WO2004/023639.